This study examined how speed was related to movement accuracy in terms of impulse variability in a timing task. The scaling of kinetic parameters in impulse resulted in the kinematic error function in a nonlinear fashion. Three experiments investigated the effect of force parameter scaling, time parameter scaling, and scaling of both parameters, The results showed that the speed itself was not sufficient enough to explain movement spatial accuracy. Spatial accuracy was proportional to the impulse variability multiplied by movement time. Timing accuracy was sensitive to the rate of force production in addition to movement time. It was also discussed that why the speed-accuracy relation was different from that of Fitts` aiming paradigm.